Cathode



Patented May 6, 1947 ZAZMM CATHODE GeorgeffT. Rayel'chiSan BrunofCalifI,assgnorto .A

Inc., SanBrunofCalyifd, a

- corporation of California A"Application-Ap1'ilv18, 1944, SerialNo.531,631 4Clams.. (Cl. Z50-i275) :My invention relates tofa cathodefon-vacuum tubes It? isamong theobjects of my inventiontogprovide acathode having improved electron emissive characteristics and superiorphysical properties.

The invention possesses other objects and features of advantage, some ofwhich, with the foregoing, will be set forth in the followingdescription of my invention. It is to be understood that I do not limitmyself to this disclosure of species of my invention, as I may adoptvariant embodiments thereof within the scope of the claims.

Referring to the drawing:

Figure 1 is a vertical sectional view showing an electron tube having afilamentary cathode embodying my improvements; and

Figure 2 is an enlarged cross-sectional view of the improved filamentwire.

Figure 3 is an enlarged sectional View of an indirectly heated cathode.

In terms of broad inclu-sion, my cathode comprises a core of materialhaving good electron emissive properties, such as thorium, clad with arefractory metal, such as tantalum; the refractory metal sheath beingpreferably carburized. The method comprises first forming the cathode ofthe improved material, and then carburizing it. The cathode is mountedin an envelope with other suitable electrodes, the envelope beingevacuated in any suitable manner.

In greater detail, and referring to the drawing, my cathode 2 isillustrated as lamentary type comprising a helical wire welded top andbottom to a pair of leads 3 sealed to stem ll of envelope 6. It isunderstood however that the cathode may be of any suitable shape orstructure, either directly or indirectly heated, as will be apparent tothose skilled in the art.

Other components of the tube chosen for purposes of illustration includea tubular anode or plate 1 and a cage-type grid 8. The plate issupported on a lead 9 sealed to the upper end of the envelope by abracket Il secured to plate cap I2. Grid 8 is supported on rod I3 sealedto stem 4 by brackets I4, one of these rods serving as a lead. A base I5cemented to the bottom of the envelope carries prongs Il connected tocathode leads 3 by conductors i8 and `to grid lead I3 by a conductor I9.

While I have shown my cathode in a triode combination, it is understoodthat it may be embodied in other tube types such as a diode ormulti-grid tubes.

Now going to the cathode or thermionic emitter element from whichelectrons are evaporated. As.

, f2. shown-1 in Figure l2, the ,-lam'ent .wire.comprises a compositestrand havingaeorelclad'withia: sheath 2 2; .Corezliisof .a materialhavinghgood electron emissivity, which would include :generallythos-ematerials havingv la. g Alow f work. function such as thorium, uraniumand bariumythorium being preferred. In the filament illustrated the coreis a solid rod composed essentially of metallic thorium, preferablyhaving a diameter about half that of the overall wire diameter. This hasthe advantage of being able to draw the final composite wire in anordinary wire machine.

If desired however the core may be of powdered metallic thoriumcompressed into the tubular outer sheath. Also, the core may be acomposition or alloy of metallic thorium with another metal, or acompound such as thorium carbide. In any case however the core is adistinct homogenous body forming the central zone of the wire, andprovides within the lament wire an ample reservoir of the highlyemissive or active material.

Sheath 22 is of a refractory metal such as tantalum or molybdenum.Tantalum is preferred for several reasons, one being that it i-s ductileenough to be readily drawn as a sheath over the core in a wire machine.Tantalum also provides a good medium through which the thorium is ableto migrate to the surface of the wire, from which surface the electronsare emitted.

, mosphere containing a ordinary thoriated tungsten Sheath 22 is alsopreferably carburized. Ihis is accomplished by heating the cathode in anathydrocarbon, -similar to the process of carburizing an ordinarytungsten filament, whereby carbon is taken up by the sheath in the formof tantalum carbide. This carburization further improves the emissiveproperties of the cathode.

My improved cathode is superior to the usual thoriated tungsten filamentwherein nely divided thoria (thorium oxide) is distributed throughout atungsten wire. In making this wire it is diicult to control thedistribution of the thoria, resulting in a bad manufacturing problem andnonuniformity in the final product. This is overcome in my cathodeelement. Furthermore my improved emitter element provides a reservoir ofthe active agent, insuring longer life of the filament. Still further,my improved lament has greater mechanical strength and is less brittlethan an ordinary th'orlated tungsten filament. This is very importantbecause many tubes are lost during manufacture and subsequent operationbecause of filament breakage.

improved emitter element is also adaptable for use in an indirectlyheated type oi cathode. As shown in Figure 3, the element is formed as asheet with a core layer 26 of metallic thorium interposed between sheathlayers 21 of a refractory metal. Tantalum is particularly preferred asthe sheath in this case because of its ductility in rolling andotherwise fabricating the composite sheet. The sheet may then be shapedas a tube or cylinder for encircling a heater 28.

My improved cathode is not to be confused with composite elements in thepast wherein a layer of material containing barium or strontium oxide isformed on the surface, or where the inner portion of the element is of arefractory metal having thorium oxide distributed in it.

I claim:

1. A cathode for electron tubes comprising a thorium core clad withtantalum.

2. An indirectly heated cathode comprising a tubular emitter elemento!sheet material having a core layer of thorium interposed between sheathlayers of tantalum. 3. 'A sheet material for cathodes comprising a corelayer composed essentially of metallic thorium interposed between sheathlayers of a refractory metal.

4. An indirectly rheated cathode for electron tubes comprising a,tubular emitting element consisting essentially of tantalum and thorium,and a heater disposed within said element.

GEORGE T. RAYFIELD.

REFERENCES CITED The following references are of record in the iile ofthis patent:

UNITED STATES PATENTS Number Name Date 1,663,547 Gero Mar. 27, 19281,695,819 ONeill Dec. 18, 1928 1,961,122 Lowry May 29, 1934 1,917,991McCullough July 11, 1933 K 1,894,059 Ruiiley Jan. 10, 1933 2,091,554Mendenhall Aug. 31, 1937 1,948,445 Lederer Feb. 20, 1934 1,957,486Becker May 8, 1934 2,140,534 Mahoney Dec. 20, 1938 2,107,945 Hull et alFeb. 8, 1938 2,175,345 Gaidies et al. Oct. 10, 1939

